The present invention relates generally to the art of hydraulics, particularly hydrostatic wheel drive systems, and more particularly relates to an improved auxiliary hydrostatic drive system in which the motors are automatically destroked whenever the auxiliary hydrostatic drive system is in a neutral condition and the motors are driven mechanically by the wheels.
In order to obtain additional traction, many agricultural and industrial tractors and similar vehicles are provided with an auxiliary hydrostatic drive for the normally nondriven steerable wheels. An example of such an auxiliary hydrostatic drive system is disclosed and claimed in U.S. Pat. No. 3,458,005 which issued on July 29, 1969 to D. I. Malm et al. Such auxiliary hydrostatic drive systems are generally employed only during periods when the load on the vehicle is great and the vehicle is moving at a relatively slow speed. During periods when the vehicle is moving at a relatively high speed, the motor is disconnected from the source of fluid pressure and, during these periods, some precaution must be taken or the high speeds at which the motors are driven can cause damage to the pistons and/or cam. In previous auxiliary hydrostatic drive systems, damage has been prevented during periods of nonuse through the use of pressure responsive clutches or through the use of variable displacement motors of the swashplate-type in which the swash plate can be returned to the neutral position. Proposals have also been made to pressurize the drive chambers of the motors from some external source so that the pistons are held off the cam.